1. Field of the Invention
The invention relates to a combustion device as well as to a method of providing low NOx and low CO combustion with substantially separate inputs for fuel and combustion air into a combustion chamber, wherein the entire or most of the combustion air is fed to the combustion chamber in continuous steps at several points in the chamber.
As used herein, the term fuel is intended to connote substances which react exothermally with oxygen and which are in a gaseous or vaporous state at ambient temperature and/or when fed into the combustion chamber. The term fuel further includes liquid or pulverized substances suspended in air, vapor and/or waste gas as a carrier gas. In this context, the term combustion air includes gas and/or vapors having an oxygen content sufficient to ensure a stable combustion of the selected fuel. The combustion air may contain waste gases. The term combustion zone as used herein is intended to include the spatial area in which the combustion takes place.
2. The Prior Art
In burners of the kind known from German published patent specifications DE-OS 4,419,345 and DE-OS 4,231,788 with separate feeding of fuel and combustion air into a combustion chamber, the combustion air is usually fed coaxially of the fuel injection. For this purpose, a fuel jet is generated near the mouth of the combustion device. The combustion air is fed peripherally of the fuel jet and outside of the flame region, through a substantially annular distributor, wherein the distributor is placed in the proximity of the fuel nozzle and substantially coaxially with respect to the fuel nozzle. Because of the significant space between the combustion air distributor and the flame region and, more particularly, the core of the flame, a uniform mixture of fuel and combustion air or a mixture composed of predetermined shares cannot be achieved in practice when operating this type of combustion device. In an attempt to reduce this disadvantage, the combustion air is segregated into primary air and secondary air, whereby locally limited peak values of oxygen concentration are lowered and the automatic control of stoichiometric ratios during combustion may thus be improved to some extent. The principal or main disadvantage of this type of combustion device, namely the unsatisfactory automatic control of the stoichiometric ratios of fuel and combustion air, is that the resultant formation of harmful and noxious substances of contaminants and pollutants, such as nitric oxides and carbon monoxide, can only be reduced at relatively high expense. A cause of this disadvantage is that the input of combustion air extends only over a relatively small spatial area of the combustion zone. Therefore, the stoichiometric ratios during combustion are essentially determined solely by the difficulty of controlling convection in the combustion zone. An attempt to reduce this disadvantage by special installations which provide for more intensive turbulence of fuel and combustion air does, however, entail a larger consumption of energy due to increased pressure losses.
Another method of reducing the formation of NOx in the combustion chamber of combustion devices not providing for partial premixing comprises injecting the combustion air and fuel at a high velocity into a combustion zone preheated to about 950xc2x0 C. This is, however, an energetically and structurally complex solution, and pyrotechnically it is of little interest since it leads to long flames and does not result in an optimum mixture.
Frequently, a multi-stage input of combustion air is carried out to improve combustion and to lower the emission of harmful substances, such as pollutants and contaminants, in combustion devices not calling for premixing. Such a proposal is, for example, the subject of the combustion device according to German laid-open patent specification DE OS 4,041,360. This combustion device with a horizontal burner pipe provided at its upper surface with a plurality of gas exit openings comprises, within so-called jet-flow rods, additional openings above the primary air input for feeding of secondary air. The flow rods are intended to cool the flames. The thermal load and stress on such jet-flow rods is, however, very high so that only high-temperature resistant materials can be employed for these jet-flow rods. Moreover, an optimum automatic control of the combustion device as to harmful substances is rendered substantially more difficult at different thermal load levels because the ratio of primary air to secondary air quantities can be changed only within narrow limits. In particular, the input of secondary air into the upper flame zone is insufficient in the region of the full thermal load.
Attempts have been made to reduce these drawbacks by separate automatic controls of the secondary air input or by partial premixing, which may require expanding the two-step air input to a three-step or four-step air input. A combustion device of the kind operating in this manner has been described in German laid-open patent specification DE OS 4,142,401. The combustion device utilizes premixing and is operated substantially below stoichiometric levels of oxygen. The oxygen which is lacking for combustion is supplied only at a noticeable distance from the combustion mouth at one or several sites, whereby the direction in which the oxygen is injected must not be the same as, and parallel to, the main flow direction of the combustion gases. This method undoubtedly improves the operation of large size industrial furnaces, such as cylindrical rotary kilns, drum-type furnaces and the like, even though it is relatively difficult to control because of the complicated flow guidance of the combustion air which must be tuned to, or adjusted in accordance with, the geometry of the furnace walls. This method is, however, too expensive for the operation of compact combustion devices with lower thermal output rates. Moreover, this method suffers from the general disadvantage that in essence the combustion air is supplied to the area of the combustion zone in which the flame temperature is relatively high.
The advantages of a multi-step air supply are also applied in a special variant of burners without premixing and provided with a combustion chamber which expands conically in the direction of the flame, wherein the combustion chamber forms a diffuser and provides more intensive mixing of fuel and combustion air. Fuel and primary combustion air are fed into the combustion devices or burners of this type (see German laid-open patent specification DE OS 3,600,784) by the diffuser and are burned in said combustion devices. Secondary combustion air is additionally fed through wall openings in the diffuser in a direction radially of the flame. However, the length of the diffuser cannot be arbitrarily extended because the diffuser would then excessively shield the flame which would impair the transfer of heat to the furnace wall. Since the length of the flames can substantially surpass the length of the diffuser in case of higher thermal output rates, this means that insufficient secondary combustion air is fed to the region of the tip of the flames particularly at higher heat output rates. The effect as regards the emission of pollutants from the combustion device or burner is unfavorable.
In combustion processes of this method, the hottest flame zones are always in the interior of the diffuser and cause the walls of the diffuser to glow red hot. This is disadvantageous for two reasons: First, the glowing leads to an increased formation of environmentally hazardous nitric oxide due to the increased temperature and, second, special temperature resistant materials are required for the walls of the diffuser. Overall, the input of combustion air is restricted to relatively small spatial areas of the combustion zone in this kind of combustion device and, moreover, the relatively small spatial areas exhibit very high flame temperatures.
Another kind of air distribution as disclosed by U.S. Pat. No. 1,247,740 is utilized to improve the mixing of combustion air and fuel. The combustion air is fed into a mixing chamber through a plurality of openings in an elongate rounded wall disposed within a furnace chamber. This mixing chamber is encased by a second elongate rounded wall surrounding the first wall, and is closed at its head section by a sealed connection of the two walls. The resultant hollow, double-walled, cylindrical chamber remains open at its bottom section and is connected at its bottom section to an annular opening for the supply of fuel. The second (outer) wall of this double-walled annular cylinder is also provided with openings at which the air-gas mixture is ignited. The combustion occurs directly at these openings as well as at the surface of the outer wall, and many individual flames are formed. A substantial disadvantage of such combustion resides in the required special and expensive materials which must be capable of withstanding the high temperatures and thermal stresses; nevertheless they may have a limited useful life.
Even though the spatially multi-stage air supply provides for a sub-stoichiometric mixing zone in the bottom section which at an increasing air ratio gradually converts to a super-stoichiometric level, control of the mixing ratios cannot be ensured as complete mixing of fuel and combustion air is quickly achieved in the double-walled enclosed restricted chamber of the annular cylinder before ignition occurs at the openings of the outer wall. The combustion also suffers from the disadvantage of flames being formed before the mixing process has been completed. The reduction effect which is important for lowering the nitric oxide emission thus does not occur.
In addition, the exhaust gas discharge from the burner chamber cannot be utilized in combustion technologies relating to heating furnaces and industrial furnaces since it cannot insure an effective transmission of heat to the material to be heated.
Further disadvantages of this double-walled burner structure result, in addition to its disposition within the burner chamber, from its complicated structure.
Another way of improving the state of the mixture comprises an additional mixing chamber serially connected ahead of the combustion chamber which results in a combustion device or burner of enlarged dimensions. As a premixer combustion device or burner it suffers from all the disadvantages inherent in such devices. Such a combustion device with very intensive premixing is described, for example, in German laid-open patent specification DE-OS 3,915,704, which combustion device is of an extremely complex structure. Its multimember mixing channels require large quantities of energy in order to compensate for the pressure loss inherent in them. Furthermore, the mixing channels are difficult to access and are, therefore, difficult to clean.
The state of the art may be summarized as follows: There is a tendency in the construction of combustion devices to utilize a multistage supply of combustion air into the combustion chamber in order better to control the stoichiometric ratios during combustion and thereby to meet the current stringent governmental standards as to economical and ecological combustion. Continued adherence to such arrangements would, however, lead to relatively complicated burner structures with a plurality of combustion air distribution lines penetrating the furnace walls or any boiler shell disposed in the combustion zone, as well as additional jet-flow rods for cooling the flames. Such solutions are neither safe to operate nor suited for a compact construction.
Another developmental tendency makes use of the principle of surface combustion in order to achieve a good mixture, complete combustion, and low emission of hazardous substances. Accordingly, the air-gas mixture is distributed over the entire surface of the burner body, protruding into the burner chamber, through a plurality of openings, and is ignited therein. Combustion occurs directly at the surface and leads to red hot glowing of the surface. The combustion air is either mixed completely with the fuel before entering into the burner body, or it is input by means of a plurality of openings in the inner wall of a double-walled cylindrical combustion device structure into the cylindrical annular space enclosed between the inner wall and the outer wall, where the combustion air is mixed with the fuel. Subsequently, the mixture is ignited at the surface of the outer burner wall. All surface burners operating according to this principle require the use of expensive materials and suffer from particular complications during assembly of the burner in the furnace chamber. In addition, their useful life and their range of application are limited to low-efficiency ranges and to gaseous fuels.
A further development line in the construction of combustion devices or burners makes use of the vacuum pressure generated by the flow velocity of the flame gas in order to draw in secondary, tertiary, etc. combustion air. However, this principle requires the flame gases to flow at a predetermined velocity by a diffuser wall provided with suction openings for the combustion air. Therefore, the volume of combustion air drawn per unit of time cannot be changed independently of the flame parameters. Even though a compact structure facilitates limiting the space available for the formation of flames by the diffuser wall, such construction suffers from the following substantial disadvantage:
The suction openings for the combustion air are disposed in a zone of very high flame temperature which leads to increased formation of environmentally hazardous nitric oxides.
The object of the invention is, therefore, to provide for a structurally simple combustion device suitable for a compact structure with substantially separate inputs of fuel and combustion air into the combustion chamber and which results in a combination of low NOx and CO emission as well as in intensified heat transfer between the flame/exhaust gas and the wall of the heat sink, wherein the combustion air is input into larger flame regions in as many stages as possible.
The object leads to the following subordinate tasks:
The quantity of combustion air fed per unit of time is metered in such a way that lambda number ranges of the mixture of fuel and combustion air preset in the combustion chamber are realized.
Decrease of thermal loads on the components groups to the combustion air feed and flame cooling as well as providing for the use of economical materials for the component groups.
Elimination of impairments of heat transfer between flame and wall of the heat sink otherwise resulting from diffusers and other means for mixing the fuel-combustion air streams.
Structuring the geometry of the combustion zone and exhaust gas zone so as to complement the walls of the heat sink.
According to the present invention, the object is accomplished by a combustion device having the features herein set forth in greater detail, as well as by a method of operating the combustion device in the most efficient way possible.
The basic concept of the invention, which also concerns the method of operating the combustion device, comprises the following: About 70 to 100 vol-% of the total quantity of combustion air supplied is furnished by one or more combustion air distributor units or diffusers in a substantially radial direction into a chamber permeated by the flame and disposed between the outer wall of the fire box and the contour of the combustion air distributor unit. The number of openings per surface unit and their cross-section distributed over the contour of the combustion-air distributor units are selected such that combustion air enters into the combustion zone at a preset flow rate. This facilitates control of the stoichiometric ratios of the mixture of fuel and combustion air. Furthermore, a predetermined curve of the range of lambda-numbers may be realized at the metering site between the base and the tip of the flame.
In contrast to the combustion air, fuel is fed into the combustion zone only in the vicinity of the base of the flame disposed at the portion of the combustion-air distributor unit by means of one or more rows of nozzles disposed around the combustion air distributor units.
If the throughput of air is less than 100%, the remaining air required for the combustion, i.e., 0 to about 30 vol-%, is mingled with the fuel before entering the combustion zone. The admixture of this portion of the combustion air increases the impulse of the fuel, improves the mixing of the fuel and the combustion air, and leads to a faster attainment of the ignition limit. This results in a drastic reduction of the NOx values.
The advantages of this concept are that combustion is initially at sub-stoichiometric oxygen values and that it achieves stoichiometric or super-stoichiometric oxygen values at gradually increasing air flow only a short distance before the tip of the flame where it brings about complete combustion. Temperature peaks or spikes are suppressed over the entire range of the flame, and the formation of harmful substances (NOx and CO) is drastically reduced. Feeding the combustion air in this manner yields the advantageous effect of the flame being blown away from the combustion-air distributor unit so that no direct combustion occurs at the surface thereof. This lowers the thermal load of the combustion air distributor units, especially since the combustion air flowing through provides additional cooling thereof.
A further advantageous result of combustion air feeding according to the invention, particularly in the case of large surface combustion air distributor units, is that they lead to simultaneous cooling of the flame whereby the formation of NOx is reduced. In addition, the use of large surface combustion air distributor units of suitable structural shape facilitates determination of the geometry of the combustion zone. For that reason, an essential function of the combustion air distributor unit is seen in the fact that the size of the combustion chamber is influenced in a significant way by a proper selection of the dimensions of the combustion air distributor units. In summary, a low thermal load of the combustion air distributor units is also achieved at varying outputs of the combustion device or burner since because of increasing combustion air throughput the cooling effect increases with increasing output of the combustion device.
The contour of the combustion air distributor units may be embodied in many different structures. Depending on the geometry of the furnace or the boiler space, an appropriate configuration of the combustion air distributor units may lead to an optimization in respect of NOx and CO emissions and in respect of heat transfer.
Additional advantageous embodiments of the invention relate to the structure of the linear arrays of nozzles for the fuel supply. For optimally maintaining preset ranges of values regarding the air lambda-number, it has been found to be particularly effective to direct the fuel jets of a given array and/or neighboring arrays to different longitudinal zones of the combustion air distributor units. At least some of the mentioned nozzles are preferably set at an angle relative to each other to impart a helical drift or swirl to the fuel. Furthermore, the combustion air distributor units and/or the fuel nozzles preferably are exchangeable for optimally adjusting their parameters to predetermined burner outputs.